Efficient and selective hydrosilylation of carbonyl compounds catalyzed by iron acetate and N-hydroxyethylimidazolium salts

Article abstract of DOI:10.1002/adsc.201100606

Aromatic aldehydes, along with aryl alkyl, heteroaryl alkyl, and dialkyl ketones were efficiently reduced to their corresponding primary and secondary alcohols, respectively, in high yields, using the commercially available and inexpensive polymeric silane, polymethylhydrosiloxane (PMHS), as reducing agent. The reaction is catalyzed by in situ generated iron complexes containing hydroxyethyl-functionalized NHC ligands. Turnover frequencies up to 600 h-1 were obtained.

Full text of DOI:10.1002/adsc.201100606

UPDATES
DOI: 10.1002/adsc.201100606
Efficient and Selective Hydrosilylation of Carbonyl Compounds
Catalyzed by Iron Acetate and N-Hydroxyethylimidazolium Salts
Elina Buitrago,a Fredrik Tinnis,a and Hans Adolfssona,*
a
Department of Organic Chemistry, Stockholm University, the Arrhenius Laboratory, SE-10691 Stockholm, Sweden
Fax : (+46)-8-154-908; e-mail: hansa@organ.su.se
Received: August 1, 2011; Revised: October 13, 2011; Published online: January 12, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100606.
Abstract: Aromatic aldehydes, along with aryl
alkyl, heteroaryl alkyl, and dialkyl ketones were ef-
ficiently reduced to their corresponding primary
and secondary alcohols, respectively, in high yields,
using the commercially available and inexpensive
lations.[7] One of the drawbacks with transition metal-
catalyzed transformations, especially when applied in
the production of pharmaceuticals and fine chemicals,
is the inherent toxicity associated with several of the
metals. Hence, catalytic applications using environ-
mentally benign and non-toxic transition metals are
highly advantageous. With regards to the latter, iron
complexes often show great biocompatibility and in
recent years there has been an increase in studies em-
ploying iron-based catalysts in various applications.[8]
Brunner first reported the iron-catalyzed hydrosilyla-
tion of carbonyl compounds in the early 1990s,[9] how-
ever, it was not until rather recently that other reports
on this particular transformation appeared in the liter-
ature. The groups of Nishiyama,[10] Gade,[11] Chirik,[12]
Tilley,[13] and Beller[14] have individually presented ef-
ficient iron-based protocols for the hydrosilylation of
polymeric
silane,
polymethylhydrosiloxane
(PMHS), as reducing agent. The reaction is cata-
lyzed by in situ generated iron complexes contain-
ing hydroxyethyl-functionalized NHC ligands. Turn-
over frequencies up to 600 hÀ1 were obtained.
Keywords: aldehydes; carbenes; hydrosilylation;
iron; ketones
The catalytic reduction of carbonyl compounds is aldehydes, ketones and imines. The catalyst systems
most frequently performed using hydrogenation with used in these studies are all based on N- and P-donor
molecular hydrogen, or via transfer hydrogenation in ligands. Recently the group of Sortais and Darcel
2-propanol or formic acid.[1,2] An alternative to these showed that efficient hydrosilylations of aldehydes
methods is the use of the combined hydrosilylation/ were catalyzed by the neutral piano-stool complex
ACTHNUTRGNEUNG
hydrolysis protocol.[3] The latter method offers a [Cp(IMes)Fe(CO)(I)].[15] In addition, Royo and co-
number of advantages over other reduction systems, workers demonstrated that similar iron complexes
where safety issues are most important. The silane re- containing a tethered N-heterocyclic carbene (NHC)
agents used in hydrosilylations are often shelf-stable ligand catalyzed the reduction of aldehydes under hy-
and can easily be activated by a number of different drosilylation conditions.[16] The latter two studies dem-
classes of catalysts. In particular, the transition metal- onstrate that N-heterocyclic carbenes, which are fre-
catalyzed hydrosilylation of aldehydes and ketones, quently used as ligands in various catalytic applica-
followed by hydrolysis of the initially formed silyl tions,[17] also are effective in iron-catalyzed hydrosily-
ethers, is a convenient protocol for the selective gen- lations.
eration of primary and secondary alcohols. There are
We have recently demonstrated that in situ generat-
several synthetically useful metal catalysts developed ed iron complexes formed from iron(II) acetate and
for this particular reaction. The dominant strategy has commercially available NHC precursors (e.g., [IPr]
been to employ low-valent group 8–10 transition [HCl]) catalyze the hydrosilylation of ketones using
metals,[4] where especially rhodium(I) complexes have the inexpensive and stable polymethylhydrosiloxane
been thoroughly studied.[5] Moreover, catalysts based (PMHS) as hydride source (Scheme 1).[18]
on early transition metals such as titanium, zirconium
The optimized reaction conditions for the hydrosi-
and rhenium,[6] as well as group 11 and 12 metals like lylation of ketones are as depicted in Scheme 1. In a
copper and zinc have been used in carbonyl hydrosily- survey of different ligand precursors we found that
Adv. Synth. Catal. 2012, 354, 217 – 222
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217

Elina Buitrago et al.
UPDATES
Scheme 1. Hydrosilylation of acetophenone catalyzed by an
in situ generated iron complex with [IPr]ACTHNUTRGNEUG[N HCl].
Scheme 2. Preparation of the salts [HEMIM]
AHCTUNGTRENNUNG
[DMHEMIM][I].[21]
ACHTUNGTRENN[UNG OTf] and
the imidazolium salt [IPr]
ACHTUNGERTN[NUNG HCl] gave the most active
catalyst. Moreover, for the deprotonation of the
NHC-precursor n-BuLi was employed, and the reac- under basic conditions to yield 2. The methylation of
tion was best performed in THF at 658C. Unfortu- 2 was performed using methyl triflate. [DMHEMIM]
nately the catalytic activity is rather low and the typi- [I] was synthesized in a two-step sequence starting
cal reaction time to reach full conversion of the start- from 1-(1-ethoxycarbonylethyl)imidazole (3), which
ing material is 16–18 h. We were therefore interested was prepared from ethyl-2-bromoisobutyrate and imi-
in evaluating other less sterically hindered carbene dazole according to the procedure by Bellemin-La-
precursors, which possibly could lead to increased cat- ponnaz, Gade and co-workers.[20] Treatment of ester 3
alytic activity.
with LiAlH4 gave alcohol 4, which subsequently was
The set of NHC precursors that we anticipated N-methylated using methyl iodide to generate the
would be easy to obtain and to evaluate in the iron- imidazolium salt.
catalyzed hydrosilylation are presented in Figure 1.
Simple imidazolium salts such as 1,3-dimethylimid- ployed in the hydrosilylation of acetophenone, initial-
azolium hexafluorophosphate ([DMIM][PF6]), and 1- ly using the optimized conditions found for the
(2-hydroxyethyl)-3-methylimidazolium triflate Fe(OAc)2/A[IPr][HCl] catalyst system (Scheme 1). As
([HEMIM][OTf]) are easily prepared in a straightfor- seen in Table 1, the reduction of acetophenone using
The NHC precursors shown in Figure 1 were em-
A
ACHTUNGTRENNUNG
A
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ward fashion. The latter compound, which has been a combination of iron acetate and [DMIM]ACHTUNRGTNEUNG[PF6] as
employed as an ionic liquid,[19] was chosen since it can the catalyst gave the same result as when the [IPr]
act as a potential bidentate ligand and thereby form a [HCl] ligand was employed (Table 1, entry 1). Gratify-
complex of higher stability. Furthermore, the dimethyl ingly, the use of [HEMIM]ACTHNUTRGNE[NUG OTf] as ligand precursor
analogue of HEMIM (DMHEMIM) was included generated a significantly more active catalyst, and full
since the introduction of substituents in the a-position conversion was observed already after 15 min using
to the imidazole could lead to enhanced stability of 2.5 mol% catalyst loading (Table 1, entry 2). Decreas-
the carbene ligand.
ing the amount of catalyst to 1 mol% resulted in a
1,3-Dimethylimidazolium
hexafluorophosphate somewhat longer reaction time, however, full conver-
([DMIM]
sequence where 1-methylimidazole initially was react- The high activity of the iron catalyst generated from
ed with iodomethane, and thereafter treated with po- [HEMIM][OTf] prompted us to evaluate the reaction
ACHTUNGTRENNUNG[PF6]), was prepared in a two-step reaction sion was obtained after only 30 min (Table 1, entry 3).
AHCTUNGTRENNUNG
tassium hexafluorophosphate to facilitate the ion ex- at lower temperatures. Performing the hydrosilylation
change. The preparation of imidazolium salts at room temperature indeed resulted in full conver-
[HEMIM]
ACHTUNGTRENNUNG
Scheme 2. [HEMIM]AHCTUNGTRENNUNG
zole, which initially was treated with 2-chloroethanol somewhat more sterically hindered imidazolium salt
[DMHEMIM][I] also showed high activity in the re-
duction of acetophenone (entry 5), however, the reac-
tion time to reach full conversion using 1 mol% of
catalyst was slightly longer in comparison to the use
of the [HEMIM]ACTHNUTRGNEUNG[OTf] ligand precursor. Obviously
the iron complexes formed from the two hydroxy-sub-
stituted imidazolium salts showed significantly higher
catalytic activity as compared to what we previously
Figure 1. Potential NHC precursors.
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Adv. Synth. Catal. 2012, 354, 217 – 222

Efficient and Selective Hydrosilylation of Carbonyl Compounds Catalyzed by Iron Acetate
Table 1. Hydrosilylation of acetophenone catalyzed by in situ generated iron complexes.[a]
Entry
Ligand precursor
[DMIM][PF6]
Reaction time [h]
Temperature [8C]
Conversion [%][b]
1
G
G
18
0.25
0.5
18
1
2
18
2
65
65
65
22
65
65
65
65
>99
>99
>99
>99
>99
–
2
A
ACHTUNGTRENNUNG
3[c]
4
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
5[c]
6
ACHTUNGTRENNUNG
5
–
7[d]
8[e]
–
–
A
R
[a]
AHCTUNGERTG(NNUN OAc)2 (2.5 mol%), imidazolium salt (3 mol%), n-BuLi (3 or 6 mol%), acetophenone (1 mmol),
THF (3 mL) and PMHS (3 equiv.). Hydrolysis of the initially formed silyl ether was performed using NaOH in MeOH
for 2 h.
[b]
[c]
[d]
[e]
Conversion determined by GLC analysis.
A 1 mol% catalyst loading was used on a 2-mmol reaction scale.
No imidazolium salt was added.
No FeACHTUNGTRENNUNG(OAc)2 was added.
observed for catalysts generated from commercial
ACHTUNGTRENiNGNU midazolium unit. Attempts to gain structural infor-
NHC precursors. To investigate the importance of the mation on the catalytically active iron complex were
hydroxy functionality, we performed the hydrosilyla- made, however, no conclusive results have so far been
tion with imidazolium salt 5 as ligand precursor.
obtained.
Compound 5 was obtained by initial treatment of 4
As seen in Table 1, the activity of the novel catalyst
with TsCl, which thereafter was converted to the azo- formed from iron(II) acetate and [HEMIM]
lium salt using methyl triflate (Scheme 3). As seen in obviously significantly higher than what we previously
Table 1, no conversion of acetophenone was observed observed using the [IPr][HCl] ligand. The former cat-
ACHTUNGTRNE[NUNG OTf] is
AHCTUNGTRENNUNG
after two hours using the same reaction conditions as alyst system reaches full conversion in 30–40 min
for the hydrosilylation performed with [HEMIM] (TOF=200 hÀ1) whereas the latter catalyst require
[OTf] (entry 6). The result using the O-protected al- overnight reaction time in order to fully convert the
cohol shows that the presence of a free OH in the starting material to product (TOF=10 hÀ1). The hy-
ligand precursor is most important for high catalytic drosilylation of ketones is typically slower in compari-
activity.
son to the corresponding reaction on aldehydes, and
(OAc)2 used in all experiments using the Fe(OAc)2/A[HEMIM][OTf] catalyst for the
The purity of the Fe
G
E
N
ACHTUNGTRENNUNG
presented in Table 1 is 99.995% based on trace metal reduction of benzaldehyde resulted in an observed
analysis. Even though this metal precursor is of high TOF=600 hÀ1 (vide infra). A comparison of the reac-
purity, the analysis shows that it contains 0.4 ppm tion profiles of the two catalytic systems is shown in
copper. Copper salts are known to catalyze hydrosily- Figure 2. As seen in the plot in Figure 2, the hydro-
lations of carbonyl compounds.[7] To rule out the pos-
sibility that the reaction is copper-catalyzed, we per-
formed a hydrosilylation of acetophenone with the
equivalent of this amount of CuACTHNUTRGNENUG(OAc)2 as catalyst.
This experiment resulted in no product formation. In
addition to the above experiments we performed hy-
drosilylations without adding any imidazolium salt or
iron acetate, respectively (Table 1, entries 7 and 8).
These experiments resulted in no conversion of aceto-
phenone which shows that the active catalyst is unde-
niably an iron complex containing the hydroxyethyl-
Figure 2. Reaction profiles for the hydrosilylation of aceto-
phenone using catalysts based on Fe(OAc)2, and [IPr][HCl]
[OTf] ( ), respectively.
G
ACHTUNGTRENNUNG
&
^
Scheme 3. Preparation of imidazolium salt 5.
( ), and [HEMIM]ACHTUNGTRENNUNG
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219

Elina Buitrago et al.
UPDATES
Scheme 4. Optimized conditions for the hydrosilylation of
carbonyl compounds catalyzed by Fe
ACHTUNGRTEN(NGNU OAc)2 and [HEMIM]
[OTf].
sis reactions are presented in Table 2. As outlined in
Table 2, aromatic, a,b-unsaturated and synthetically
useful heterocyclic aldehydes react rapidly under
these conditions to generate the corresponding pri-
mary alcohols after silyl ether hydrolysis (entries 1–4).
Furthermore, sterically hindered ketones were readily
reduced and gave the corresponding secondary alco-
hols in high isolated yields (entries 5, 6 and 15). Ke-
tones bearing functional groups that are susceptible
to reduction, or can be replaced under reductive con-
ditions, remained intact under these reaction condi-
tions. Hence, the ester- and nitrile-functionalized ace-
tophenones 14 and 16 were reduced in high yields
(entries 8 and 10), as was also the fluoro-substituted
ketone 15 (entry 9). Gratifyingly, the iron/hydroxyalk-
yl-NHC catalytic system worked well for synthetically
useful heteroaromatic ketones. Acetylpyridines react-
ed smoothly and gave the corresponding alcohols in
good yields (entries 11 and 12). 2-Acetylthiophene
Figure 3. Carbonyl compounds screened in the hydrosilyla-
tion.
Table 2. Hydrosilylation of aldehydes and ketones (see
[OTf].[a]
Figure 3) catalyzed by FeACTHNUTRGENN(GU OAc)2 and [HEMIM]AHCTUNGTRENNGUN
A
ACHTUNGERTN(NUNG OAc)2/
Entry Carbonyl com-
pound
Reaction time
[min]
Yield
[%][b]
ACHTUNGTRENNUNG
A
E
ACHTUNGTRENNUNG
1[c]
2[c]
3[c]
4[c]
5
7
8
9
10
30
60
60
30
60
120
120
150
60
60
30
60
60
180
120
>99
>99
>99
>99
81
85
85
85
78
93
91
79
90
98
85
92
starts to turn over. The reason for the observed
higher reactivity of the hydroxyethyl-functionalized
imidazolium catalyst is not known. However, the fact
that an active catalyst is generated fast indicates that
the alkoxide formed upon deprotonation of the
HEMIM group rapidly coordinates to iron and facili-
tates the formation of the carbene complex. Most
likely the ligand coordinates to the active iron catalyst
in a bidentate mode.
10
11
12
13
14
15
16
17
18
19
20
21
22
6
7
8[d]
9
10
11
12
13
14
15
16
Furthermore, the reduction of acetophenone was
performed using the FeCAHTUNGTERN(NNUG OAc)2/AHCTUNRTGEGUN[NN HEMIM]ACTHNUGTREN[GUNN OTf] cata-
lyst system in the presence of 1,1-diphenylethene
(10 mol%), a known radical trap. This experiment re-
sulted in full conversion of acetophenone within
90 min. The result indicates that the mechanism for
the iron-catalyzed hydrosilylation performed under
these conditions is not involving radical species.
To investigate the scope of the reaction using the
FeACHTUNGTRENNUNG(OAc)2/ACHTUNGTRENNUNG[HEMIM]ACHTNUGERTN[NUGN OTf] catalyst system, a set of
different carbonyl compounds (Figure 3) was subject-
ed to the optimized reaction conditions shown in
Scheme 4. The results of the hydrosilylation/hydroly-
[a]
General conditions: FeACTHNUTRGENN(UG OAc)2 (1 mol%), [HEMIM]ACHTUNGTRENNUNG[OTf]
(1.1 mol%), n-BuLi (2.2 mol%), substrate (2 mmol),
THF (5 mL) and PMHS (3 equiv.). Hydrolysis of the ini-
tially formed silyl ether was performed using NaOH in
H2O for 1 h.
[b]
[c]
[d]
Isolated yields.
1
Conversion determined by H NMR.
Hydrolytic work-up using TBAF, see ref.[22]
220
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Adv. Synth. Catal. 2012, 354, 217 – 222

Efficient and Selective Hydrosilylation of Carbonyl Compounds Catalyzed by Iron Acetate
and 2-acetylfuran were readily reduced and the prod- Acknowledgements
ucts were isolated in excellent yields (entries 13 and
This work was financially supported by the Swedish Research
Council, the Carl Trygger Foundation, the K & A Wallenberg
Foundation, and the Magn. Bergvall Foundation.
14). In addition to the abovementioned substrates, we
examined the reduction of 3’-nitroacetophenone, and
found that this particular ketone reacted poorly
giving only 20% yield of the corresponding alcohol
after 24 h. Nitroacetophenones are known to be prob-
lematic in iron-catalyzed hydrosilylations as shown,
for instance, by Royo and co-workers.[16] Overall, with
the exception of 3’-nitroacetophenone, the catalytic
system performed well for all examined substrates
and the alcohols were obtained in good to excellent
yields within a rather short reaction time (0.5 to 3 h)
using a catalytic loading of 1 mol% iron acetate.
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Experimental Section
General procedure for the hydrosilylation of carbonyl
compounds catalyzed by FeACHTUNRGTNEUNG(OAc)2 and HEMIM·OTF
The iron source, Fe
ACHTUNGTRENNUNG
ligand precursor [HEMIM]AHCTNUGTRENNNUG
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the Supporting Information.
[22] Upon completion of the initial hydrosilylation (TLC),
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